In this specification, where a document, act or item of knowledge is referred to or discussed, this reference or discussion is not an admission that the document, act or item of knowledge or any combination thereof was at the priority date, publicly available, known to the public, part of common general knowledge; or known to be relevant to an attempt to solve any problem with which this specification is concerned.
Nitrogen is an important plant nutrient. Urea (CO(NH2)2) represents more than 40% of the total nitrogen fertiliser applied to agricultural crops worldwide. However, urea is readily degraded in soil by the “urease” group of enzymes, which catalyses the reaction of urea with water to produce gaseous ammonia and ammonium ions (referred to as “urea hydrolysis”). The ammonia is readily volatile from the treated soil; leading to a loss of up to 60% of the applied urea as a result of this enzymatic hydrolysis.
In order to delay this hydrolysis, “urease inhibitors” have been applied to urea based fertilisers in an attempt to reduce the rate of urea hydrolysis and subsequent loss of ammonia. Examples of such urease inhibitors include the N-alkyl thiophosphoric triamides such as N-(n-butyl)thiophosphoric triamide (NBPT). However, NBPT is a waxy, sticky, heat-sensitive and water-sensitive material and so particular formulations are required to minimise decomposition during storage and distribution. Examples of such formulations include:                a concentrated solution of an N-alkyl thiophosphoric triamide in a solvent mixture of glycols (eg propylene glycol) and liquid amides (eg N-methylpyrrolidone) (see for example international patent application no WO 97/22568);        a mixture comprising a thiophosphoric acid triamide and a compound containing an amino group having a boiling point of more than 100° C. (see for example international patent application no WO 2009/079994); and        a liquid composition containing a phosphoric or thiophosphoric triamide derivative and one or more of esters of hydroxyacids, heterocyclic alcohols, cyclic esters of carbonic acid and esters of dicarboxylic acids (see for example international patent application no WO 2010/072184).        
The existing urease inhibitors formulations currently used in agriculture (e.g. the commercial product called Agrotain™ (trade mark of Phosphate Resource Partners Limited Partnership, registered in some countries)) suffer from a number of disadvantages in use, including:                limited storage stability of the treated urea;        health and safety concerns regarding the solvents used in the formulation; and        ecotoxicology concerns regarding the effect of solvents used in the formulation once in the aquatic and terrestrial environment.        
Therefore, there is a need for an improved urease inhibitor formulation which addresses at least one of these disadvantages.